HIGH CAPACITY/LOW NOx RADIANT WALL BURNER

Abstract

A method for operating a radiant wall burner to heat a radiant surface adjacent a combustion zone, said method comprising: providing a fuel lean combustible fuel-air mixture; causing the fuel-air mixture to flow outwardly from a main nozzle, into said combustion zone and generally accross said radiant surface in a circular pattern which essentially surronds said main nozzle in a radial direction; causing the fuel-air mixture to flow outwardly from said main nozzle at an initial velocoty which exceeds the flame speed of the mixture, whereby a detached round flame is created; providing a secondary fuel at a location in the furnace on an opposite side of said zone from said radiant surface, said secondary fuel constituting a substantial portion of the total fuel provided to said combustion zone by said fuel-aire mixture supply system and said secondary fuel nozzle system.

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE NATURE OF THIS INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFOMED. 1

BACKGROUND OF THE INVENTION
Field of the Invention /." • , ^
The present invention relates to the field of industrial burners and in particular to radiant wall burners which operate to heat the surrounding portions, of a wall of a furnace or the like, which often consist of a burner tile, and these heated surrounding portions then distribute heat by radiation in the furnace. Even more particularly, the invention relates to methodology and apparatus whereby the efficiency and capacity and NOx reduction capabilities of radiant burners is enhanced. The State of the Prior Art
Reduction andVor abatement of NOx m radiant burners has always been a desirable aim. Moreover, it has always been a desirable aim in the industry to increase the heatproduction capacity of individual radiant bumers withou.t detrimentally affecting NOx production. Radiant burners which use a primary premix produced by inducing a
flow of air with fluid fuel are known, but previous burners have not been capable of producing fuel-air premixes containing less than about 80% of the total fuel, Such
premixes combust at high temperatures resulting in excessive production of NO. and
other contaminants. Moreover, the amount of secondary fuel available for other purposes
such as carrying flue gas into the flame has been extremely ln^tedjbecause thej priim Accordingly the
industry has needed means for improving the efficiency of burners for radiant burner, applications such that the primary pre-mix is leaner in fuel whereby a large mass of air
is available during the initial combustion to" reduce the combustion temperature and a large amount of secondary fuel is available for circulating in the furnace space away from the flame so as tojremix with a large amount of flue gas to further reduce combustion temperatures. The industry has also needed radiant burners with greater heat production capacities.
SUMMARY OF THE INVENTION
The present invention alleviates the problems discussed above and enhances radiant burner installations by providing a high capacity, low N0X radiant wall

burner assembly wherein the primary fuel-air premix has a much higher air content and
a correspondingly much lower fuel content than previously thought possible fey those skilled in the art. The burner of the invention is also capable of generating greater. amounts of heat than previously known burners. In accordance with the concepts and" principles of the invention, a high capacity radiant burner is provided which includes a burner tube structure comprising an elongated burner conduit having spaced inlet and outlet ends. The conduit is adapted and arranged for directing a fuel lean gaseous mixture comprising a portion of the total fluid fuel to be; combusted and oxygen therealong from the inlet end to the outlet end. A main burner nozzle is provided at the outlet end of the conduit, and such burner nozzle has a central axis, a wall extending around acentrally located^3^1 t-stherein, and a downstream end spaced from the outlet
end of the conduit. The main burner nozzle is arranged and adapted forreceiving the fuel lean fuel-air mixture from"the conduit in the cavity redirecting the same without substantial recirculation and with rrjinimal pressure drop through aplui^jty of apertures
in mewail and into a combustion zone in a direction transverse to the axis and at a
velocity which is greater than the flame speed of the gaseous rnixture. The apertures are distributed around the wall, whereby the fuel-air rnixture directed into the combustion zone through the apertures is generally in the form of a round flat partem which is detached from the nozzle, surrounds the wall and extends outwardly across a radiant surface of a burner tile. Ideally, the fuel lean gaseous rnixture includes all of the oxygen needed for combusting the total fuel delivered to the furnace.
The burner of the invention also includes an elongated fuel tube that extends in a direction generally parallel to the axis of the nozzle. The fuel tube has s
downstream end portion and a secondary fuel nozzle including at least one secondary fuel port is positioned on the downstream end portion of the fuel tube. Each secondary fuel port is located and arranged so as deliver secondary fuel to a location in the furnace •
which is on the opposite side of the round flat pattern from the radiant surface and is
sufficiently remote from the combustion zone to permit the same to be came intermixed
with flue gases before entering the combustion zone.
• In accordance with the invention, the elongated fuel tube may be located externally of main fuel. nozzle.and each secondary fuel port may be located and arranged so as to deliver secondary fuel at a velocity and in a direction such that at least
a portion of the secondary fuel pierces the pattern to reach the proper location described
3 "■ ■

above. Alternatively, the elongated fuel tube may extend through the main feel nozzle and protrude through the downstream end thereof to deliver the secondary feel directly to the location which is on the opposite side of the fuel-air pattern from the radiant surface.
Preferably, the burner tube structure may comprise a venturi tube which uses a flow of the gaseous fuel to induce a flow of air, whereby $6 create the fuel lean fuel-air mixture. Ideally, the mixture may comprise a nuxture of a gaseous fuel and air. In another form of the invention, the burner tube structure may comprise a plurality of venturi tubes arranged for parallel flow, each of the Venturis being adapted and arranged to use a flow of the gaseous fuel to induce a flow of air, whereby to generate the mixture as an ultra fuel lean mixture of fuel and air.
In a more specific sense, the high capacity, low NO, radiant wall burner according to the invention may include an elongated nozzle arrangement adapted for installation in a central passageway of a refractory burne tile inserted in a wall of a furnace adjacent a combustion zone. The tile may preferably have a radiant surface surrounding the passageway and located adjacent the cpmbustion zone. The nozzle arrangement may include an elongated burner tube including an elongated downstream portion configured to extend through the passageway and an elongated upstream portion, suchporh^nsmayhaverespechVecenfrallym^posedjlongituQinaUy extending axes. The nozzle arrangement may also include a fuel-air mixture supply system providing afsource of a fuel lean combustible fuel-air mixture for introduction into the burner tube, an upstream end of the upstream portion of the burner tube being connected in fluid communication with the fuel supply system for receiving the fuel lean combustible fuel-air mixture, the burner tube providing a conduit for flow of the fuel lean combustible fuel-air mixture therealong from the upstream end to a downstream end of the downstream portion of the burner tube.
The nozzle arrangement of the invention may further include a main nozzle positioned at the downstream end of the downstream portion of the burner tube adjacent the radiant surface, the main nozzle having an internal chamber that is in fluid communication with the downstream end of the downstream portion of the burner tub*; for receiving the fuel lean combustible fuel-air mixture flowing along the tube. The main nozzle is arranged and configured to redirect the fuel-air mixture in the chamber and. cause it to flow without substantial recirculation in a direction radially outwardly relative .
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to the axis of the downstream portion of the burner tube, into the combustion zone, and
generally across the radiant surface. The main nozzle has a wall extending around the
chamber and a scries of radially extending openings in (lie wall. The openings ure
arranged and configured to dispense the combustible fael-airmixture in a radial direction
at an initial velocity which exceeds the flame speed of the mixture and in a circular
pattern which essentially surrounds the nozzle in a radial direction, whereby a detached
round fiame is created when the mixture is combusting Finally, the burner arrangement
may desirably include a secondary fuel nozzle system including an elongated fuel tube
extending longitudinally of the downstream portion of the burner tube and having at least
one fuel gas port disposed and arranged to direct a flow of secondary fuel to a location
in the furnace on an opposite side of the combustion zone from the radiant surface. The
secondary fuel constitutes a substantial portion of the total fuel provided to the
combustion zone by the fuel-air mixture supply system and the secondary fuel nozzle
system. . . ,
In accordance with a highly preferred form of-the invention, the fuel-air supply system of the burner may comprise an ejector including, a fuel inlet connectable to a source of pressurized fluid fuel, a fluid fuel spud connected in fluid communication with the inlet and positioned for ejecting fluid fuel through a space in fluid communication with a source of air, and a generally bell-shaped fitting mounted at the upstream end of the upstream portion of the burner tube. The bell-shaped fitting has a
moumpositioned for receiving the ejected fluid fuel and air carried along with it and directing the same into the upstream end of the burner tube.
In one form of the invention, the axes of the pojtijo^sjoX&e^bjraexlube 1 a%^^f may be superimposed whereby the burner tube is essentially straight. Thus, the main nOz^El^te^umer tube and the ejector are in essential ahgnment along the superimposed axes. In an alternative form of the invention, the axis of the upstream portion may be

disposed at an angle relative to the axis of the downstream portion, whereby the main
nozzle and the downstream portion of the burner tube are disposed in essential alignment along the axis of the downstream portion, and the ejector and the upstream-portion may be burner tube are disposed in essential alignment along the axis of the upstream portion. In one form of the invention, the elongated fuel tube may be located outside the mainjiozzle. Preferably, in this fornTof the invention, the secondary fuel nozzle system may include a plurality of elongated fuel tubes located outside the main
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nozzle. Desirably, the ports of the secondary fuel tubes are each configured and positioned to cause at least a portion of the secondary fuel to pierce the fuel-air mixture pattern and reach the desired location in the furnace without combusting.
-
In another form of the invention, where the secondary fuel nozzle extendi! through the main nozzle and an eductor is used to premjx the primary fuel-air mixture, the secondary fuel system may desirably be arranged to bypass the eductor. This may be done as discussed above by arranging the axes of the upstream and downstream portionsof the burner tube at an angle. Alternatively, the secondary fuel system may include a segment of tubing which extends laterally through a wall of the downstream portion of the burner tube, such segment being connected in fluid communication with an upstream end of the fuel tube.
In ahighly preferred form of the invention, the openings in the nozzle wail may desirably comprise elongated slots which extend in a direction tiiat is essentially parallel to the axis of the downstream portion of the burner tube. Preferably, the wall of the nozzle may comprise a series of circumferentially spaced bars presenting the slots therebetween, the bars having rounded surfaces adjacent the charnber to inhibit the formation of recirculation zones in the chamber. Ideally, the burner may include an internal baffle having a generally bell-shaped downstream portion located in the chamber. The bell-shaped portion may have an outer, circumferentially extending edge disposed adjacent the wall. Additionally, the slots may have an upstream end and a downstream end, and the outer edge of the bell-shaped portion may be located closer to the upstream end of the slot than to the downstream end pf the slot; Ideally, the outer edge of the bell-shaped portion may be located approximately one-fourth of the distance from the upstream end of the slot to the downstream end of the slot. Furthermore, the slots may
6

desirably have upstream end surfaces that slope in a direction of fluid flow to inhibit the formation of recirculation zones in the chamber.
In a preferred form of the invention, the fuel-air mixture supply system and the secondary fuel system may be arranged such that the amount of the secondary fuel constitutes more than about 20 %, desirably at least about 30 %&nd ideally at least
J".
about 50 to 60 % of the total fuel provided to the combustion zone. Ijri a further preferred form of the invention, the relationship between the velocity that the primary fuel-air mixture leaves the slots and the flame speed of the mixture is such that the upstream extremity of the detached flame is positioned between about 1 inch and | inches from the nozzle to make sure that the radiant tile is heated evenly.
In accordance with another preferred ajspect of the invention, when the: axes of the upstream and downstream portions of the burner tube are disposed at an angle, the burner tube may desirably-include a curved portion which interconnects the downstream and upstream portions thereof, and the secondary fuel system may include a segment of tubing which extends through a wall of the curved portion of the burner . tube. This segment of tubing is connected in fluid communication with an upstream end. of the fuel tube. Ideally, the arrangement is such that the segment of tubing and the fuel tube extend essentially along the axis of the downstream portion of the burner tube and the eductor is offset at an angle. With this arrangement, the eductor for the primary fuel air mixture is bypassed by the secondary fuel system, and the overall longitudinal dimensions of the burner are reduced.
The invention fiirther provides a method for operating ahigh capacity, low NOx radiant wall burner. The method comprises (1) delivering a flow of a fuel lean combustible mixture comprising a portion of the total fuel to be combusted and air in a radial direction from an elongated nozzle having a central axis to a conibustion zone surrounding the nozzle in the form of a round flat pattern which surrounds the wall and at a composition where the flame speed of the rnixture is lower than the velocity of the • mixture as the latter exits the nozzle, the combustion zone being adjacent a radiant face of a burner tile; (2) igniting the mixture to create a round flat detached flame which surrounds the nozzle in a radial direction and is located adj acent the radiiant face; and (3) providing a supply of secondary fuel at a location on the opposite side of the flame from the radiant face and spaced far enough away from the flame so that the secondary fuel becomes intermixed with flue gas before it enters the flame.
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More specifically, the method may desirably comprise (1) providing a fuel lean combustible fuel-air mixture; (2)causmgthefael-akrnixtureto flow outwardly from a main nozzle, into the combustion zone and generally across the radiant surface in a circular pattern which essentially surrounds the main nozzle in a radial direction; (3) causing the fuel-air mixture to flow outwardly from the main nozzle at an initial velocity which exceeds the flame speed of the mixture, whereby a detached round flame is created when the mixture is combusting; and (4) providing a secondary fuel at a location in tb e furnace on an opposite side of the zone from the radiant surface, the secondary fuel constituting a substantial portion of the total fuel provided to the combustion zone by the fuel-air mixture supply system and the secondary fuel nozzle system.
In accordance with the invention, the secondary fuel desirably constitute s more than about 20 %, preferably constitutes at least about 30 % and ideally constitutes at least about 50 to 60 % of the total fuel provided to the combustion zone.
In one form ofthe invention, the secondary fuel is provided!at the location on the opposite side of the primary fuel-air partem using a secondary fuel nozzle which extends through the main nozzle. Alternatively, the secondary fuel is provided at the location using a secondary fuel nozzle which emits a jet of fuel that pierces the pattern without combusting. ■ BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view, partly in cross-section, illustrating a high capacity, low NOx radiant wall burner which embodies the concepts and principles of the invention and associated accessories;
FIG. 2 is an enlarged view, partly in cross-section, of certain major components of the burner of Fig. 1;
FIG. 3 is a cross-sectional view taken long the line 3-3 of Fig. 2;
FIG. 4 is a side elevational view, partly in cross-section, illustrating another embodiment of a high capacity, low NOx radiant wall burner which embodies the concepts and principles of the invention and associated accessories;
FIG. 5 is a cross-sectional view taken long the line 5-5 of Fig. 2;
FIG. 6 is a view that is similar to Fig. 5 except the end cap for the main nozzle has a slightly different shape;
FIG. 7 is an enlarged detail view of the circled portion of Fig. 6;
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FIG. 8 is a detail view similar to Fig. 7, except for the configuration of the entrance portion of the slots;
FIG. 9 is a side elevational view, partly in cross-section, illustrating yet another embodiment of ahigh capacity, low NOx radiant wall burner which embodies the concepts and principles of the invention and associated accessories;
FIG. 10 is a side elevational view, partly in cross-section, illustrating a further embodiment of a high capacity, low NOx radiant wall burner which embodies the concepts and principles of the invention and associated accessories;
,.: FIG. 11 is an enlarged cross-sectional view illustrating the downstream portions of a secondary fuel nozzle which is useful in connection with the various embodiments of the invention;
FIG. 12 is a schematic, elevational view of a further embodiment of a burner which embodies the concepts and principles of the invention;
FIG. 13 is a schematic, elevational view of illustrating the operational principles of the burner of Fig. 1;
FIG. 14 is a schematic, side elevational view illustrating a yet another high capacity, low NOx radiant wall burner which embodies the concepts and principles of me invention and associated accessories; .
FIG. 15 is a schematic, elevational view iUustrating the operational principles of another burnerwhich embodies the concepts and principles of the invention;
FIG 16 is an enlarged cross-sectional view illustrating the details of the primary fuel delivery spud and the secondary fuel delivery system of the burner of Fig. 9; and
FIG 17 is an enlarged detail view of the circled portion of Fig. 1.0.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE
INVENTION
The invention provides a high capacity, low NO, radiant wall burner. In one important aspect, the invention focuses on the provision of a high capacity, low NOx radiant wall burner which employs a fuel-lean fuel-air mixture to fuel the primary flame. Fuel lean primary fuel-air mixtures assist in improving turn down ratios, at least in part because fuel lean fuel-air mixtures are slow burning and have a reduced combustion velocity. Fuel lean primary fuel-air mixtures also operate to reduce, and perhaps
9

eliminate completely the need for secondary air, which often increases the production of NOx. Leaner fuel-air combustion mixtures, however, tend to reduce the overall capacity of the main burner and it has previously been thought that the highest capacity possible for such burners having an outside nozzle diameter of about 5.5 inches is no m6re than about 1.2MMBtu/hr. In accordance with the invention, however, capacities above about 2.0 MMBtu/hr have become routine without adversely"affecting NOx output. In fact, when the burners of the invention are used to achieve high outputs, NOx levels have often been improved. The burners of the invention, due to the increased capacity and reduced flame speed, also provide uniform heating of the radiant tiles and a reduced tendency for flashback, even when the fuel is predominantly hydrogen. In this latter regard, the type of fuel used by the burner is not intended to be a critical lirnitation, and in accordance with the concepts and principles of the invention, the burner of the invention may be used with any sort of available combustible fluid fuel or fuel mixture, including, but .not limited to, natural gas, hydrogen, mixtures of natural gas and hydrogen/ etc.
Que embodiment of a burner which is based on tiae concepts and principles of the invention is illustrated in Fig. 1, where it is identified by tne reference numeral 20. The burner 20 desirably consists generally of an elongated nozsle arrangement 22 which includes an elongated burner tube 24, a main burner nozzle 26, a secondary fuel system 28, and a fuel-air mixture supply system .30 which desirably provides, a fuel lean primary combustible fnel-air mixture to the burner tube 24 for delivery to the nozzle 26 for ultimate distribution to a combustion zone 32 that generally surrounds nozzle 26 in a radial direction. As shown at least partly in Fig. 1, the burner 20 includes all of .the conventional components which are usually associated with industrial burners, including a muffler 34, an air control 36 and a fuel gas manifold arrangement 38, including an inlet 40, for receiving and delivering a fluid, preferably gaseous fuel to the burner 20 from a supply source (not shown in the drawings), and a primary fuel supply line 39. For convenience, the secondary system 28 may also be connected to inlet 40 as shown. The gaseous fuel may desirably be natural gas or a mixture of natural gas and hydrogen.
The burner tube 24, which provides a conduit for conducting a flow of a fuel lean mixture of fuel and air from the supply system 30to the nozzle 26, includes, as shown on Fig. 1, an elongated upstream portion 42 and an elongated downstream portion 44. The portions 42,44 have respective, centrally disposed axes 46, 48 which extend

longitudinally therealong. The downstream portion 44 is configured to extend through a central passageway 54 provided in a refractory burner tile 56 arranged in the wall 58 of a furnace or the like. "
Tile 56 has a radiant surface 6Q which surrounds passageway 54 and is
adjacent combustion zone 32 so as to be heated by combustion occurring in zone 32
during operation. As shown in Fig. 1, mesunfeoeJJOj^yJ^^
other shapes are well known to those skilled in the art, thus, as can be seen from Fig.
1, mam nblizIelRns^ end 52 of downstream portion 44 of
the burner tube 24 adjacent radiant surface 60.
With further reference to Fig. 1, it can be seen that the burner tube 24 may preferably include a,curved portion (or elbow) 62 interconnecting portions 42 and 44. Accordingly, the axes 46, 48 are disposed at an angle, with the nozzle 26 and the downstream portion 44 aligned along axis 48, and "with the upstream portion 42 and the supply system 30 aligned along axis 46.
As can be seen viewing Fig. 2, nozzle 26 is provided with an inwardly curved, generally trumpet-shaped end cap 64 having a centrally located hole 66 therein. Nozzle 26 afso mcludes a wall 68^which extends^ompetely therearound, Thus, the end cap 64 and the wall 68 define a chamber 70 inside nozzle 26 which is in fluid communication with the downstream end 52 of downstream portion 44 of burner tube 24 so as to receive the fuel lean fuel-air mixture from the burner tube 24. As can be seen viewing Fig. 2, the end cap 64 is convex relative to chamber 70- Thus, the nozzle 26 is configured and arranged to redirect the fuel lean primary fuel-air mixture without substantial recirculation, and cause it to flow outwardly away from nozzle 26 in a" direction radially outward relative to axis 48. Thus, the primary mixture flows into combustion zone 32 and across the radiant surface 60. To this end, the nozzle 26 is provided with a circumferentially extending series of radially extending openings 72, which preferably are in the form of elongated, axially extending slots. These elongated slots 72, which extend in a direction that is essentially parallel to the axis 48, are preferably defined by a series of circumferentially spaced bars 74 as can be seen viewing Fig.3. Desirably, in one very important application of the invention, the nozzle 26 may be cylindrical and approximately 5.5 inches in outer diameter. The bars 74 may be approximately one-half inch wide in a radial direction so that the inside diameter of chamber 70 is approximately 4.5 inches. The nozzle 26 may have approximately 90
W

slots, each of which is about 2 inches long and about 0.055 inches wide. With regard to the foregoing, while these dimensions; etc. are preferred for an existing application, it is to be understood that the dimensions of the nozzle and the slots are not critical features of the invention. For example, in retrofit applications, the diameter of the nozzle may ; generally be limited by the size of an existing nozzle passageway and the size and shape of the slots may be limited by furnace capacity and fuel characteristics and parameters. In new furnace construction there is more freedom and there is no particular limitation on nozzle diameter. Regarding slot size and shape, suffice it to say that sufficient area must be provided to handle the volumetric flow rate of the fuel-air mixture and provide an escape velocity which exceeds the flame speed- of the mixture and positions the detached upstream end of the flame such that the radiant surface is heated evenly. As will be appreciated by those stalled in the art, the optimum dimensions may depend upon such variables as the characteristics and parameters of the available fuel, the heating capacity of the furnace and the total volume of the primary fuel-air mixture, and as a result, slot dimensions for any given application may often need to be detennined empirically so as to minimize pressure drop and the presence of recirculation zones within the nozzle.
The secondary fuel system 28 may mclude a length of tubing 76 which is connected through a fitting 78 permitting tubing 76 to enter tube 24 through elbow 62. inside tube 24, tubing 76 is"connected in flnid conniiunication with the upstream end of an elongated secondary fuel tube 80 that extends longitudinaHy of downstream portion 44 of tube 24 along axis 48. As can be s&m in Fig. 2, fuel tube 80 extends through chamber 70 and has a downstream end 82 which protrudes through hole 66 in end cap 64. With reference to Fig. 11, it can be seen that the end 82 is provided with one or more ports 83 to direct the flow of secondary fuel outwardly into the furnace space. Tube 80 may also be provided with an internal orifice 85 to control the flow of the secondary fuel which desirably constitutes a substantial portion of the total fuel supplied to the combustion zone.
With reference now to Fig. 5, it can be seen that the burner assembly 22 " may desirably include abaffle 84 that is mounted within chamber 70 of nozzle 26. Baffle 84 may be provided with a series of tabs 86 (only one is shown) which may be attached to the inner surface 88 of wall 68 by welding or the like to properly center and position baffle 84. Baffle 84 may preferably have a bell shaped downstream portion 89 having
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an outer circumferentially extending edge 90 that is positioned adjacent the inner surface
88 of wall 68. Slots 72 desirably each have an upstream end 92 and a downstream end
94, and it is preferred that the axial position of baffle 84 is such that edge"90 is closer to
the upstream ends 92 than to the downstream ends 94. Ideally, the edge 90 may be
positioned approximately one-fourth of the distance from the upstream ends 92 to the
downstream ends 94. That is to say, when the slots 72 are 2 inches longIthe edge 90 may
desirably be positioned one-half inch in an axial direction from the ends 92 of the slots
72. With regard to the axial position of edge 90, it is to be appreciated by those skilled
in the art that this also is not a critical limitation on the scope of the invention. Suffice
it to say in this regard that the optimal axial position of the edge 90 is simple that
position where both pressure drop and the development jofjre^irculation zones ire
minimized. ,
The fuel-air mixture supply system 30 may be in the form of a conventional ejector or venturi 95 which includes a primary nozzle or spud 96 for ejecting gas jets through a space 98 that is in communication with a source of air and a venturi inlet bell 100. These components are mounted inside muffler 34 in Fig. 1 and cannot be seen. However, the spud 96, the space 98 and the inlet bell 100 are shown schematically in Fig. 13 which also illustrates the operation of the burner 20. The details of an appropriate spud 96 are also illustrated in Fig. 17 where it can be seen that the spud 96 may desirably include an internal fuel chamber 118 which is connected to fuel supply line 39 (See Fig. 1) and a plurality of, preferably three, jet orifices 120. The orifices 120 which may be drilled in an end plate 121 of spud 96, are sized to provide an appropriate flow rate to the nozzle 26. The spud is connected to a supply of pressurized gas which gas is ejected through jet orifices 120 and through space 98 where air is entrained therein. The fuel gas and the entrained air are injected in a generally parallel direction relative to axis 46. The motive energy from the fuel gas provides the energy used to aspirate the surrounding combustion air into the inlet bell 100 and through the venturi section of th e burner. The mixture of fuel and entrained air, which desirably is a fuel lean mixture, then flows into and is received by the open end or mouth 99 of the inlet bell 100.
The upstream portion 42 of the burner tube 24 may include a venturi throat portion 50 and a diffuser portion(5l) The inlet bell 100 is designed to provide a j smooth, uniform flow path for the combustion air from space 98 into the venturi throat 50. The venturi throat 50, which is located jnst downstream of the inlet bell 100, consists.
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essentially of a straight tube. The design parameters of the tube, and particularly its length and diameter, are important because they play a critical role in the aspiration performance of the combustion air. The downstream end of the venturi throat is attached . to the diffuser 51 .The diffuser 51may preferably be in the form of an elongated conicid section that provides a gradual transition from the throat 50 to the long radius elbow 62. The long radius elbow 62 provides two functions. First, it allows the venturi to be offset so as to conveniently position the secondary fuel system 28 to bypass the throat section 50. This design configuration provides a substantial improvement in air aspiration performance as compared to designs where the secondary fuel riser is located along the centerline of the throat. This design increases the aspiration performance of the combustion air that results in a lower flame temperature providing a substantial reducation
of NOx emissions. Secondly, the elbow 62 provides a method for reducing the overalllenght of the burner. In many applications the overall length of the burner is limited to furnace space constraints. Using elbows with different angles allow designs to meet specific customer needs. The downstream portion 44, which may be in the form of a tube with a specific length, is attached to the downstream end of the elbow. "When the air-fuel mixture exits the long radius elbow the flow patterns of the air-fuel mixture are highly skewed. The downstream portion 44 allows the gas flow profile to become evenly distributed before the same enters the burner nozzle 26. An even flow distribution through the burner nozzle is important for good flame quality.
In operation, the slots 72, in association with the baffle 84, dispense the fuel lean primary fuel-air mixture without substantial recirculation and with minimal pressure drop in a radial direcation at an initial velocity that exceed the flame speed of the mixture. This desirable flame speed condition may be detenrrined empirically depending upon the total flow area provided by the slots, the total flow volume of the fuel-air mixture, and the pressure of the latter. The slots 72 are also arranged so as to direct the primary fuel-air mixture radially outward from the nozzle 26 so as to form therefrom, in zone 32, a circular pattern 102 which surrounds nozzle 26 in a radial direction. Preferably, the fuel lean primary fuel-air mixture dispensed via slots 72 contains less than 80 % of the total fuel to be combusted in the combustion zone 32. Evenmore desirably, the fuel lean primary fuel-air mixture contains less than about.70 % of the total fuel to be combusted in the combustion zone 32. And ideally, the fuel lean primary fuel-air mixture may contain less than about 50 % of the total fuel to be
14

combusted in the combustion zone 32. As a result of the initial velocity of the mixture, the circular pattern 102 desirably provides a flame, when combustion occurs, that is. detached from the nozzle 26 and has an upstream extremity 104 that is located approximately between 1 and 3 inches from the nozzle 26.
At the same time that the primary fuel-air mixture is directed radially from nozzle 26, secondary fuel traversing the downstream end 82 of tube 80, which protrudes axially from end cap 64 of nozzle 26, is directed by ports 83 to an adjacent location 106 which surrounds downstream end 82 of fuel tube 80 within the furnace but is downstream from pattern 102 and on the opposite side thereof from radiant surface 60. This flow is illustrated by the arrows 108 in Fig. 13. As the fuel circulates through the furnace space away from the combustion zone 32, it entrains flue gases and eventually returns to the primary combustion zone 32 where it enters into the combustion reaction. This entrainment is illustrated by the arrows 110. The presence of the entrained flue gases operates to reduce flame temperature and therefore NOx production. In accordance with the invention, the secondary fuel may preferably be more than about 20 %, desirably at least about 30 % and ideally 50 to 60 % or more of the total fuel supplied to the combustion zone.
An end cap having an alternative shape is identified by the reference numeral 164,in Fig. 6. In this case, the end cap 164 is generally conical in shape. Other than the shape of the end cap 164 and the dimensions thereof, the nozzle 126 .of Fig. 6 is essentially the same as the nozzle 26 of Fig. 5. Desirably, in another very important application of the invention, the nozzle 126 may be cylindrical and approximately 3.375 inches in outer diameter. The bars 174 may be approximately one-fourth inch wide in a radial direction so that the inside diameter of chamber 170 is approximately 2.875 inches. The nozzle 126 may have approximately 60 slots 172, each of which is about 2 inches long and about 0.058 inches wide.
In both Figs. 5 and 6, the upstream end surfaces 92, 192 of the slots 72, 172 are shown as being flat and disposed in a plane which is essentially perpendicular to walls 68, 168. Alternatively, these end surfaces may be sloped in the direction of fluid flow as illustrated in Fig. 8, where the sloped end surfaces are identified by the reference numeral 292. The sloped surfaces" 292 may assist in inhibiting the formation of recirculation zones in chamber 270. In this same vein, and with reference to Fig. 3, the
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internal edges 92 of bars 74 may desirably be rounded, again to assist in the inhibition
—- " " f
of recirculation zones in chamber 70.
The main burner nozzle 26 thus includes a series of slots that allow the combustible air-fuel mixture to exit the burner nozzle 26 in a radial direction, generally parallel to the furnace wall and across the radiant surface 60 without substantial recirculation and with minimal pressure drop in the nozzle 26, The width, depth, and length of these slots may be optimized by those, skilled in the art so as to provide an appropriate exit area needed for the required burner firing capacity and toensure that the burner operates without flashback. The internal baffle 84 located inside the burner nozzle

26 is used to help redirect the air-fuel mixture in such a manner as to prevent
r
recirculation zones in the region of the burner nozzle 26. TJb[ej?rjyjentionof.reGkculation
zones near the burner nozzle 26 is important because it help^rgduce NOx emissions bj
" assisting in the detachment of the primary flame from the burner nozzle 26. Detaching
the primary flame from the main burner nozzle 26 allows more, furnace gases to be
entrained into the flame. This results in a reduction in the flame temperature that lowers
NOx emissions. Internal baffles similar to the baffle 84 are illustrated in U.S. Patent No,
, 4,702,691. However, the internal baffle 84 is used in a different manner in accordance
with the principles and concepts of the present invention. Thus, the baffle 84 is used to
l reduce the amount of energy required to aspirate the air-fUeLmbctureftrough the burner
"1 nozzle 26 by .minimizing the pressure drop and presence of recirculation zones in the
Jnozzle 26. The overall design thus provides a burner nozzle and eductor system which
is able to aspirate more combustion air resulting in a leaner primary air-fuel mixture.
Such a leaner air-fuel mixture results in a reduction of flame temperature resulting in
lower NOx emissions.
NOx emissions may be raduced even futher using the staged fuel concept described above. The staged fuel is delivered to a location in the furnace on fee opposite side of the combustion zone from the radiant tile. The fuel may be staged using a rise* that is inserted through the venturi elbow section and through the center of the burner downstream section and nozzle. A staged fuel riser protrudes through the center of the end plate of the burner nozzle. The ports of the staged fuel riserjare preferably designed so that the staged fuel is injected at a location spaced from the furnace wall and primary flame. The staged fuel mixes with furnace gases before being entrained into the primary flame. The mixing of the staged fuel with the furnace flue gases, prior to combustion,
16

reduces the flame temperature resulting in a reduction in NOx emissions. The exact ingle of injection is not critical, so long as the secondary fuel remains away from the nam combustion zone for a sufficient length of time to entrain a substantial NOx .educing amount of furnace gases. In actual practice, the secondary fuel may leave the riser therefor at an angle which, is outward, inward or parallel to the furnace wall.
An alternative embodiment of a high capacity, low NOx radiant wall burner which embodies the concepts and principles of the invention if illustrated in Fig. 4, where it is identified by the reference numeral 220. The only essential difference between the burner 220 and the burner 20 is that the upstream portion 42a of the burner tube 24a is cylindrical rather than conical. In addition, the nozzle 26a is provided with a series of holes 114 to increase the flow area for the radially directed primary fuel lean fuel-air mixture. Fig. 4 also illustrates the use of the burner of the invention in conjunction with a tile 56a having a concave or cup-shaped radiant surface 60a.
Another alternative embodiment of a high capacity, low NO, radiant wall burner which embodies the concepts and principles of the invention is illustrated in Fig. 9, where it is identified by the reference numeral 320. In the burner 320, the upstream portion 42b of the burner tube 24tus aligned axially with the downstream portion 44b. Thus, the burner tube 24b is straight In this case, the secondary fuel system 28b includes
a tubing segment 76b which extends from spud 96b through the bell shaped fitting 100b. The details of the arrangement of the spud 96b and the tubing segment 76bare illustrated in Fig, 16 where it can be seen that the chamber 118b is in direct communication with the upstream end 76b"of the tubing segment 76b. The spud 96b is provided with a plurality of primary fuel ejecting ports 120b which are arranged around upstream end 76b"of the tubing segment 76b in a location for inducing the flow of air into the upstream end 99b ofbell-shaped fitting 100b. Tubing segment 76b is connected to secondary fuel tube 80b having a downstream portion 82b provided with ports 83b. These ports 83b operate to deliver secondary fuel to the location 106b on the opposite side of the combustion zone 32b from the radiant surface 60b. A shortcoming of this embodiment, although it is fully operable in a functional sense, is that the tubing segment 76b extends through the throat of the ejector and diminishes the flow area thereof. Accordingly, as explained above, the capacity of the ejector to induce the flow of at is reduced and it is tlierefore more difficult to produce an ultra fuel lean premix using thjs embodiment.
17

Yet another alternative embodiment of a high capacity, low NOx radiant wall burner which embodies the concepts and principles of the invention is illustrated in Fig. 10, where it is identified by the reference numeral 420. In the burner 420, just like in the burner 320 of Fig. 9, the upstream portion 42c of the burner tube 24c is aligned axially with the downstream portion 44c. That is, the axes 46c and 48c are superimposed, the burner tube 24c is straight, and the main nozzle, 26 the burner tube 24c, the bell shaped fitting 100c and the ejector spud 96c are in essential alignment along the Superimposed axes 46c, 48c. Spud 96c of Fig, 10 is identified by the reference numeral 96 in Fig. 1 In burner 420, however, the problems of burner 320 are avoided" in that the secondary fuel system 28c is designed to bypass the ejector system provided by the spud 96c and bell shaped fitting 100c. To this end, the system 28c includes a secondary fuel tubing segment 76c disposed outside the upstream portion 42c of the burner tube 24c. As shown in Fig. 10, tubing segment 76c may include a straight length 116 and an angled length 118. Length 118 is disposed at an angle relative to length 116 and extends through wall 120 of downstream portion 44c. The downstream end of length 118 (not shown in Fig. 10), is connected in fluid communication with the upstream end of secondary fuel tube 80c. The tube 80c may be the same as the tube 80 depicted in Fig. 1 . It is to be noted in connection with the foregoing that the secondary fuel systems 28 and 28a of burners 20 of Fig. 1 and 220 of Fig. 4 respectively, also totally bypass the ejector system to avoid the shortcomings of the burner 320 of Fig.9.
An arrangement which is similar to the arrangement of burner 420 of Fig. 10 is illustrated schematically in Fig. 12. In the burner arrangement of Fig. 12, the secondary fuel system 28d includes a plurality of segments 76d which bypass the upstream portion 42d. Each of these segments" 76d include straight lengths 116d and angled lengths 118d. As can be seen in Fig. 12, lengths 118d extend through the WJIII 120d of downstream portion 44d, and the downstream ends 118d" of lengths 118d are connected in fluid communication with an upstream end of tube80 the downstream end 82d-of which extends through nozzle 26d and end cap 64d.
Yet another alternative embodiment of a high capacity, low NOx radiant wall burner which embodies the concepts and principles of the invention is illustrated schematically in Fig. 14, where it is identified by the reference numeral 520. The burner 520 may be essentially the same as the burner 20 of Fig. 1 in all functional respects, except that in thus case the fuel-air mixture supply system 30e which provides a fuel lean

primary combustible fuel-air mixture to the burner tube 24e for delivery to the nozzle 26e
for ultimate distribution to a combustion zone 32e, may include more than one upstream
ejector or venturi 95e. The multiple venturi system useful in connection with the burner
520 is fully describedaad illustrated in said co-pending application serial number
09/874,383 and may include a multiplicity of Venturis. That is to say, the number of
Venturis which may be combined to deliver a primary fuel-air mixture to the downstream
portion 44e, which may be in the form of a collector, may number 2 or 3 or 4 or even 8
or more, and the exact number is limited only by the physical dimensions of the space
where the burner is to be used. Suffice it to say that the use of multiple Venturis may
enable shortening of the length of the overall system and the production of ultra lean
primary fuel-air mixtures. It is also to be noted that in tjie burner 520, the centra ly
located secondary fuel system 28e fully bypasses the Venturis 95e.
A further alternative embodiment ofahigh capacity, low NOx radiant w all
burner which embodies the concepts and principles of the invention is illustrated
schematically in Fig. 15, where it is identified by the reference numeral 620. In this case,
the elongated fuel tubes 80f of the secondary fuel system 28f are disposed outside the
nozzle 26f. Each tube 80f has a downstream end portion 82f which is similar to the end
portion 82 illustrated in Fig.l 1. That is to say, each portion 82f may be provided with
one or more ports 83 f configured and positioned so that at least a portion of the secondary
fuel is delivered to the location 106f which is within the furnace but is downstream from
pattern 102f created by nozzle 26f in the manner described above in connection with the
burner 20 of Fig. 1 and which is on the opposite side of pattern 102f from the radiant
surface (not shown in Fig. 15). The secondary fuel is delivered by ports 83 f to location
106f by causing the same to be delivered.in an appropriate direction and at a sufficient
velocity to pierce through the pattern 102f without combusting so as to reach location
-■■MirtW5^iaiHii~Hjnif»""i»"PiV-"r"J-" "in r ■TI.-.TT" ■r«MTr-iii"T"--"i
106f in an uncombusted condition. This piercing flow is illustrated by the arrows 1 OSf in fig.15.As the fuel circulates through the furnace space adjacent location 106f away
from the combustion zone 32f, it entrains flue gases and eventually returns to the primary combustion zone 32f where it enters into the combustion reaction. This entrainment is illustrated by the arrows 110f. The staged fuel risers are preferably designed so that the staged fuel is injected into the"furnace at a pressure ranging from 2 to 15 psig, and at an angle from the horizontal. Part of the injected fuel mixes with the primary flame, bui. a substantial portion thereof penetrates through the primary flame envelope and into the
19

furnace downstream from the primary flame where it mixes with furnace gases before being re-entrained into the primary flame. Previously, as illustrated in U.S. Patent No. 5,180,302, similar external secondary fuel npzzles were open ended tubes, and the secondary fuel gas simply mixed with, the primary flame. In the present case, however, , the secondary gas is carefully metered by the ports 83f and accelerated by the pressure of the fuel such that piercing of the primary flame occurs.
In summation, the invention thus provides a high capacity, low NO,t, partially premixed, staged fuel burner. Preferably, the burner includes a venturi section that is optimized sufficiently to deliver a fuel lean premjxed mixture of fuel and air to the main nozzle of the burner. The main burner nozzle, which is located at the exit end of the venturi section, has radially directed exit slots which allow the combustible mixture to exit the main nozzle in a radial direction and generally parallel to the furnace wall. In accordance with the concepts and principles of the invention, the width, depth, and length of these slots are optimized to provide the appropriate total exit area necessary for the high burner firing capacity, and to ensure that the burner operates without flashback problems using fuel mixtures that may often contain high levels of hydrogen. The flam e established by the main burner nozzle is called the primary flame. The design of the exit slots of the main burner nozzle, and the use of at least one internal baffle to aid in turning the premixed fuel air flow without recirculation zones being formed, result in a flame that is normally sustained at a certain distance away from the burner. This "detachment" of the primary flame results in larger amounts of furnace flue gases being entrained into the primary flame, thus reducing the NOx emissions. The use of a fuel lean fuel-air mixture for the primary flame is an important parameter in "detaching" the flame from the main burner. The fuel lean primary gas mixture preferably falls in a range of flammabihty conditions that make it difficult for the flame to become attached on the burner tip. Supplementation of the primary flame envelope with staged fuel provides the additional fuel needed to make the combustible mixtures fall in the appropriate range for stable combustion.
In addition, the NOx emissions are further reduced with the injection of staged fuel. The fuel can be staged using side mounted risers equipped with staged-fuel tips. The fuel can also be staged using a center riser that is inserted through the venturi -burner tip assembly and protrudes through the end plate of the burner tip. Preferably, however, the fuel is staged using a secondary fuel tube which bypasses the venturi
20

portion of the burner but still passes through the main nozzle and protrudes through the . nozzle end plate.
The staged fuel may desirably be inj ected into the furnace at a location on the opposite side of the primary combustion zone from the radiant tile and at a pressure ranging from 2 to 15 psig. In addition, the secondary fuel is injected into the furnace at an angle from the horizontal and away from the primary flame. The staged fuel mixes with furnace gases before being entrained into the primary flame. Because of the way the staged fuel is injected and the pressure used in the process, the "secondary", or rather, the staged flames established are short (especially with heavier hydrocarbon fuels), well defined, and away from the furnace tile, resulting in uniform heating of the furnace.tile and wall. The center riser results in lower noise emissions, because of the use of multi pie ports to deliver primary fuel main burner tip. The use of multiple fuel ports causes a shift in the jet generated noise to higher frequencies.
Typical premixed burners do not utilize successfully so rriany technologies andbasic theories atonce to achieve high firing capacities, extremely low NOx emissions, and high stability over a wide range of operating capacities and fuels, as the new design described by this disclosure does. The new design displays the following performance characteristics:.
1. High firing capacity without increasing burner diameter;
2. Very low NO,;
3. . Short flame profiles;
4. Detached primary flame;
5. Extremely uniform tile and furnace wall heating;
6. High turndown ratios due to leaner primary fuel-air mixtures.;
7. High stability at all operating conditions;
8. Operation using fuel mixtures containing high levels ofhydrogen;
9. Low noise generation;
10. Effective and efficient operation m most commercially available tiles;
11. Utilization of staged fuel for lower NOx emissions;
12. Secondary fuel induced flue gas recirculation for lower NOx emissions;
13. Simplicity.
21

WE CLAIM
1, A burner assembly for a radiant burner comprising:
a burner tube structure (22) comprising an elongated burner conduit (24, 24a, 24b, 24c, 24e) having spaced inlet and outlet ends, said conduit being adapted and arranged for directing a fuel lean gaseous mixture comprising a portion of the total fluid fuel to be combusted and oxygen therealong from said inlet end to said outlet end;
a main burner nozzle (26, 26a, 26c, 26d, 26e, 26f) at the outlet end of said conduit, said burner nozzle having a central axis, a wall (68, 168) extending around a centrally located cavity (70, 170, 270) therein, and a downstream end spaced from said outlet end of the conduit, said main burner nozzle being arranged and adapted for receiving said mixture from the conduit in said cavity and redirecting the same through a plurality of apertures (72, 172) in said wall towards a combustion zone; and
a secondary fuel delivery system (28, 28a, 28b, 28c, 28d, 28e, 28f) including at least one elongated secondary fuel tube (80, 80b, 80c, 80f) extending longitudinally of said axis, said secondary fuel tube having a secondary fuel nozzle (82, 82b, 82d, 82f) located at a downstream end portion thereof, said secondary fuel nozzle including a secondary fuel port (83, 83b, 83f);
characterized in that
said apertures (72, 172) are so distributed around said wall that the mixture is directed through the apertures into the combustion zone (32, 32b, 32e, 32f) in a direction transverse to said axis and generally in the form of a round flat pattern that surrounds said wall (68, 168) and extends outwardly across a radiant surface (60, 60a, 60b);
said secondary fuel delivery system includes a said secondary fuel tube (80, 80b, 80c) disposed within the main burner nozzle or a plurality of said secondary fuel tubes (80f)
22

disposed externally of the main burner nozzle; and
the secondary fuel port is located and arranged so as to discharge a flow of secondary fuel causing the same to travel in a direction outwardly away from said surface (60, 60a, 60b) to a location (106, 106b) that is axially spaced from and out of contact with said combustion zone (32, 32b, 32e, 32f), whereby the secondary fuel will have an opportunity become mixed with fuel gases before undergoing combustion with excess air in the combustion zone.
2. A burner assembly as set forth in claim 1, further comprising a fuel-air mixture supply system (30) providing a source of a fuel lean combustible fuel-air mixture for introduction into the inlet end of said burner conduit (24, 24a, 24b, 24c, 24e), said fuel-air mixture supply system being arranged and configured to establish a fuel lean fuel to air ratio in said fuel-air mixture whereby the latter has a predetermined low flame speed, said apertures (72, 172) being arranged and configured to dispense said combustible fuel-air mixture in said transverse direction at an initial velocity which exceeds the predetermined flame speed of the mixture and in said pattern, whereby a round flame (102) that is detached from the nozzle (26, 26a, 26c, 26d, 26e) is created upon combustion of the mixture.
3. A burner assembly as set forth in claim 2, wherein said secondary fuel delivery system is configured and arranged such that the secondary fuel delivered through the secondary fuel nozzle (82, 82b, 82d, 82f) is pure fuel.
4. A burner assembly as set forth in claim 2 or 3, wherein said fuel-air mixture supply system (30) and said main nozzle (26, 26a, 26c, 26d, 26e, 26f) are configured and arranged such that said fuel-air mixture includes all of the air required for combusting said fuel.
23

5. A burner assembly as set forth in claim 2, 3 or 4, wherein said a fuel-air mixture supply system (30) and said main nozzle (26, 26a, 26c, 26d, 26e, 26f) are configured and arranged such that said fuel-air mixture includes no more than 55% of the total fuel combusted in said burner.
6. A burner assembly as set forth in claim 1, wherein the downstream end of the main burner nozzle has a hole (66) in it and said elongated fuel tube (80) extends axially through the cavity (70) with the downstream end portion of the secondary fuel tube projecting through said hole.
7. A burner assembly as set forth in claim 1, wherein said mixture comprises a mixture of a gaseous fuel and air, and said burner tube structure comprises a venturi tube (95) which uses a flow of said gaseous fuel to induce a flow of air, whereby to create said mixture.
8. A burner assembly as set forth in claim 1, wherein said mixture comprises a mixture of a gaseous fuel and air, and said burner tube structure (24e) comprises a plurality of venturi tubes (95e) arranged for parallel flow, each of said Venturis being adapted and arranged to use a flow of said gaseous fuel to induce a flow of air, whereby to generate said mixture as an ultra fuel lean mixture of fuel and air.
9. A burner assembly as set forth in claim 1, wherein said plurality of elongated fuel tubes (80f) located externally of said main fuel nozzle have secondary fuel ports (83f) located and arranged so as to deliver secondary fuel at a velocity and in a direction such that at least a portion of the secondary fuel pierces said pattern to reach said location (106f).
10. A high capacity, low NOx radiant wall burner including a burner assembly as set forth in claim 1, said burner tube structure being adapted for installation in a passageway (54) in a wall (58) of a furnace adjacent said combustion zone (32), said radiant surface (60)
24

being disposed at said furnace wall in surrounding relationship to said passageway, said burner tube structure (24) including an elongated downstream portion (44, 44b, 44c) configured to extend through said passageway and an elongated upstream portion (42, 42a, 42b, 42c), said portions having respective centrally disposed, longitudinally extending axes (48, 48c, 46, 46c), said burner further including a fuel-air mixture supply system (30) providing a source of said fuel lean gaseous mixture for introduction into said burner tube structure, said inlet end of the burner tube structure (24) being in fluid communication with the fuel-air mixture supply system (30) for receiving the fuel lean gaseous mixture therefrom, said apertures (72, 172) in said main burner nozzle wall (68) being arranged and configured to dispense said gaseous mixture in said transverse direction at an initial velocity which exceeds the flame speed of the mixture whereby said round flat pattern is detached from said main burner nozzle during combustion of the mixture.
11. A high capacity, low NOx radiant wall burner as set forth in claim 10, wherein said fuel-air mixture supply system (30) comprises an ejector (95) including a fuel inlet connectable to a source of pressurized fluid fuel, a fluid fuel spud (96, 96b, 96) connected in fluid communication with said inlet and positioned for ejecting fluid fuel through a space (98) in fluid communication with a source of air, and a fitting (100) mounted at an upstream end of the upstream portion of the burner tube structure, said fitting having a mouth positioned for receiving a premix comprising the ejected fluid fuel and air carried along with it and directing the same into the inlet of the conduit (24).
12. A high capacity, low NOx radiant wall burner as set forth in claim 11, wherein said axes (46c, 48c) of the upstream and downstream portions (42c, 44c) of the burner tube structure are superimposed whereby the latter is essentially straight, and said main burner
25

nozzle (26c), said burner tube structure (24c) and said ejector are in essential alignment along said axes.
13. A high capacity, low NOx radiant wall burner as set forth in claim 11, wherein the axis (46) of the upstream portion (42) of the burner tube structure is disposed at an angle relative to the axis (48) of the downstream portion (44) of the burner tube structure, whereby said main burner nozzle (26) and said downstream portion (44) of the burner tube structure are disposed in essential alignment along the axis (48) of said downstream portion of the burner tube structure, and said ejector (95) and said upstream portion (42) of the burner tube structure are disposed in essential alignment along the axis (46) of said upstream portion of the burner tube structure.
14. A high capacity, low NOx radiant wall burner as set forth in claim 10, wherein said plurality of elongated fuel tubes (80f) located externally of said main fuel nozzle have ports (83f) configured, positioned and arranged to cause at least a portion of the secondary fuel to pierce the fuel lean gaseous mixture pattern and reach said location (106f) in the furnace without combusting.
15. A high capacity, low NOx radiant wall burner as set forth in claim 10, wherein said main burner nozzle (26, 126) includes an end cap (64, 164) having a hole (66) in it, and wherein said secondary fuel tube (80) extends through said cavity (70, 170) and said downstream end portion protrudes through said hole, said port being positioned adjacent said location (106) in the furnace.
16. A high capacity, low NOx radiant wall burner as set forth in claim 15, wherein a plurality of said ports (83) are provided in said secondary fuel nozzle (82) and said location (106) in the furnace surrounds said downstream portion of the secondary fuel tube.
26

17. A high capacity, low NOx radiant wall burner as set forth in claim 10, including a radiant surface (60) is essentially flat.
18. A high capacity, low NOx radiant wall burner as set forth in claim 10, including a radiant surface (60a) that is concave.
19. A high capacity, low NOx radiant wall burner as set forth in claim 18, including a radiant surface that is cup-shaped.
20. A high capacity, low NOx radiant wall burner as set forth in claim 12 or 15, wherein said secondary fuel delivery system (28) includes a segment of tubing (76c) which extends through a wall of said downstream portion (44c) of the burner tube, said segment being connected in fluid communication with an upstream end of the secondary fuel tube (80).
21. A high capacity, low NOx radiant wall burner as set forth in claim 15, wherein said end cap (64, 164) is convex relative to said cavity (70, 170).
22. A high capacity, low NOx radiant wall burner as set forth in claim 10, wherein said apertures comprise elongated slots (72, 172) which extend in a direction which is essentially parallel to the axis (48) of the downstream portion (44) of the burner tube.
23. A high capacity, low NO,x radiant wall burner as set forth in claim 22, wherein said wall of the main nozzle comprises a series of circumferentially spaced bars (74, 174) presenting said slots (72, 172) therebetween, said bars having rounded surfaces adjacent said cavity (70,170) to inhibit the formation of recirculation zones in the cavity.
24. A high capacity, low NOx radiant wall burner as set forth in claim 22 or 23, wherein said burner includes at least one baffle (84) having a generally bell-shaped downstream portion (89) located in said cavity (70), said bell-shaped portion having an outer, circumferentially extending edge (90) disposed adjacent said wall of the main nozzle.
27

25. A high capacity, low NOx radiant wall burner as set forth in claim 24, wherein said slots (72) have an upstream end (92) and a downstream end (94) and said outer edge (90) of the bell-shaped portion is located closer to the upstream end (92) of the slot than to the downstream end (94) of the slot.
26. A high capacity, low NOx radiant wall burner as set forth in claim 25, wherein said outer edge (90) of the bell-shaped portion is located approximately one-fourth of the distance from the upstream end (92) of the slot to the downstream end (94) of the slot.
27. A high capacity, low NOx radiant wall burner as set forth in any one of claims 22 to 26, wherein said slots (72) have upstream end surfaces (292) that slope in a direction of fluid flow to inhibit the formation of recirculation zones in the cavity.
28. A high capacity, low NOx radiant wall burner as set forth in claim 10, wherein said fuel-air mixture supply system (30) and said secondary fuel system (28) are configured and arranged such that the amount of said secondary fuel constitutes more than about 20 % of the total fuel provided to the combustion zone.
29. A high capacity, low NOx radiant wall burner as set forth in claim 28, wherein said secondary fuel delivery system (28) is arranged for connection of the elongated fuel tube or tubes (80, 80b, 80f) to a source of fuel gas at a pressure of at least about 2 psig (14kPa).
30. A high capacity, low NOx radiant wall burner as set forth in claim 10, wherein an upstream extremity (104) of said flame (102) is positioned at least about 1 inch (25mm) from said nozzle and no more than about 3 inches (76mm) from said nozzle.
31. A high capacity, low NOx radiant wall burner as set forth in claim 13, wherein said burner tube structure (24) includes a curved portion (62) which interconnects said downstream and upstream portions (42, 44) thereof, and wherein said secondary fuel system
28

includes a segment of tubing (78) which extends through a wall of said curved portion of the burner tube, said segment being connected in fluid communication with an upstream end of the secondary fuel tube (80).
32. A high capacity, low NOx radiant wall burner as set forth in claim 31, wherein said segment of tubing (78) and said secondary fuel tube (80) extend essentially along the axis (48) of said downstream portion (44) of the burner tube.
33. A high capacity, low NOx radiant wall burner as set forth in claim 12, wherein said secondary fuel system (28) includes a segment of tubing (76b) that is connected in fluid communication with an upstream end of the fuel tube (80b), said segment extending through said fitting and through said spud, said spud including a plurality of orifices (120b) for ejecting fluid fuel, said orifices being arranged around said segment of tubing.
34. A radiant wall burner as set forth in claim 10 or 11, wherein said fuel-air mixture supply system is arranged and adapted for supplying in said mixture all of the oxygen needed for combustion of said total fuel.
35. A burner assembly as set forth in claim 1 or 8, wherein said fuel lean gaseous mixture includes all of the oxygen needed for combustion of the total fuel.
36. A burner assembly as set forth in claim 1, wherein said radiant surface (60) is part of a refractory burner tile (56) inserted in a wall of a furnace, and wherein said main burner nozzle (26) extends through a passageway (54) in said tile (56).
37. A high capacity, low NOx radiant wall burner as set forth in claim 11, wherein said fitting (100) is generally bell-shaped.
38. A high capacity, low NOx radiant wall burner as set forth in claim 1, wherein said main burner nozzle (26) is arranged and adapted for redirecting the gaseous mixture
29

at an initial velocity which exceeds the flame speed of the mixture, whereby said round flame is detached from the nozzle.
39. A burner assembly as set forth in claim 2, wherein said fuel-air mixture supply system comprises at least one venturi tube (95), said venturi tube being adapted and arranged to use a flow of said gaseous fuel to induce a flow of air, whereby to generate said mixture as an ultra fuel lean mixture of fuel and air.
40. A burner assembly as set forth in claim 2 or 39, wherein said fuel-air mixture supply system is arranged (30) and adapted for supplying in said mixture all of the oxygen needed for combustion of said total fuel. ,
Dated this 16th day of April, 2002